Conventional flash circuits substantially comprise the following elements:
a switching element, which periodically switches the supply voltage (battery) on and off,
a transformer, which transforms the periodically connected battery voltage into a high voltage suitable for a flash tube,
a rectifier, which rectifies the high voltage,
a flash capacitor, which is charged up to the rectified high voltage and hence stores the energy for the flash,
a trigger device, which supplies an overvoltage pulse leading to ignition of the flash tube,
and the flash tube, which after ignition converts the energy stored in the flash capacitor into light.
In said case, the switching element is connected to the remaining components in such a way that a self-sustaining oscillation is established.
Since both the flash capacitor and the flash tube are approved only for a specific maximum high voltage, it has to be ensured in some way that said limit is not exceeded.
A simple way is such that, by suitably designing the transformer, the high voltage is limited in such a way that overcharging of the flash capacitor is ruled out. This leads to an unnecessarily long charging time.
For shorter charging times the transformer has to be dimensioned in such a way that, in principle, considerably higher voltages than are approved might also be attained. In order nevertheless to prevent overcharging of the flash capacitor, a device is required which, on attainment of the maximum permissible voltage, interrupts the entire charging operation. Said device in all hitherto known circuits comprises an additional active element which, on attainment of the approved high voltage, interrupts the above-mentioned self-sustaining oscillation of the switching element Transistors, field-effect transistors, thyristors or similar components are used as active elements. Such active components add considerably to the cost of the circuit.
The object of the invention was to provide a circuit for a photographic flashlight, which
a) uses components which are as inexpensive as possible,
b) has a short charging time,
c) provides a clear indication of the charge state of the battery and in which
d) the maximum high voltage is not exceeded.
A further object is that a button need merely be pressed briefly to charge the flash capacitor.
In a circuit variant according to the state of the art, the customer has to close a switch in order to charge the capacitor. The flash capacitor is charged as long as the switch is closed. This has the advantage that directly after a flash the capacitor is immediately charged again and so the energy for the next flash is provided without any further action on the part of the customer. The drawback is, however, that the switch has to be opened by the customer when the flash facility is no longer required. Otherwise the battery would be unnecessarily depleted. To remind the customer to switch off, the switch may, for example, be fashioned in such a way that in the ON state it projects noticeably beyond the housing of the camera.
In a second circuit variant according to the state of the art, instead of the switch a button has to be pressed until a suitable display element indicates the state of readiness of the flash. This has the advantage of automatically interrupting the circuit to the battery after release of the button but is found bothersome by many customers.
In a third circuit variant according to the state of the art, a button merely has to be pressed for a short time to charge the flash capacitor. The charging operation is automatically stopped when either sufficient time to charge the capacitor has elapsed or the voltage across the capacitor has reached the required value. Said variant likewise has the advantage that the customer does not have to bother about interrupting the circuit to the battery. Even an automatic recharging of the capacitor after a flash may be realised electronically without forfeiting the above-described advantage.
In said third variant, the voltage-dependent cutoff is to be preferred and is achieved according to the invention in a specific embodiment.
It was surprisingly discovered that a behaviour comparable to cutoff by means of active components may be realised also by means of exclusively passive components. Here, by passive components are meant resistors, inductors, capacitors, diodes and similar components, the behaviour of which cannot be influenced by a control voltage or a control current. Such components are considerably less expensive.
In a preferred embodiment, as a switching element a conventional transistor is used, which connects the primary side of the transformer by its collector-emitter section to the battery. The rectified secondary current of the transformer is conducted via the base of the transistor, wherein the described self-sustaining oscillation is established. The frequency of said oscillation is dependent upon the actual charge state of the flash capacitor. If the base of the transistor is connected to its collector by an additional capacitor of suitable capacitance, then at a given specific frequency of the oscillation, the voltage across the base of the transistor is disturbed in such a way that the oscillation breaks off. As the oscillating frequency depends upon the actual voltage of the flash capacitor, it is possible to avoid overcharge by selection of suitably dimensioned components.
How the components are to be dimensioned depends in particular upon the inductance of the transformer windings and the properties of the used transistors. Any electronic engineer may, in view of the given teaching, develop suitable embodiments of the invention.
What is novel according to the invention is, that the interruption of the self sustaining oscillation is effected not by active elements, but by passive components, which disturb the conditions necessary to sustain the oscillation in dependence of the actual charge of the flash capacitor.